Assuming winforms:
Implement the MouseUp event handler as follows:
private void richTextBox1_MouseUp(object sender, MouseEventArgs e)
{
if (e.Button == MouseButtons.Right)
{
RichTextBox box = (RichTextBox)sender;
box.SelectionStart = box.GetCharIndexFromPosition(e.Location);
box.SelectionLength = 0;
}
}
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